Because the pressure in the engine when the pistons move inwards and outwards changes, Citroen fitted the oil filler tube with an ingenious mechanism to maintain a certain under pressure (is this an English word?) to keep the engine oil inside the engine, instead of the pressure build up by the movement of the pistons in the cylinders pushing it out through the gaskets and seals.
Although i didnt have any complaints about oil leakage or signs that the breather rubbers were shot, I was advised to revise the filler neck. Then you know for sure the pressure inside the engine is within the specification and the engine oil stays were it belongs.
To revise the filler I used a kit that is made by VGS in Belgium, supplied by Eendengarage Sander Aalderink. I also got some hands on support by the crew at this wonderful workshop specialized in all Citroen Type-A cars like 2CV, Dyane, Ami and Mehari.
Disassembling is pretty easy; just carefully file or sand away just enough of the folded metal that keeps the two halves together so that the two halves separate. Inside there are several rubber membranes, that can all be replaced by using the kit. I only have some pictures of all stuf disassembled.
The rubbers in my filler neck were pretty hard, and i could easily snap and break them by pushing a bit on them. So I’m glad I did it. The procedure is pretty easy and requires no special tools. A special plastic ring and a adjustable clamp is supplied to assemble the two halves together. I used a tiny bit of good quality engine gasket in between the two halves of the filler neck.
I will test the pressure later, when the engine is in the car, but I am pretty sure its going to be ok.